The Global Positioning System (GPS) is very well known as a mechanism for providing relatively accurate positioning information using small portable devices. To create a remote tracking device useful for tracking or monitoring persons GPS devices need a mechanism to transmit the location information from the GPS to a central site where a record of the person's location can be maintained. There have been several devices that have used terrestrial wireless or cellular networks coupled to a GPS engine to transmit the location data to a central repository. The GPS/cellular device can either transmit the raw GPS data over the cellular network to a central system which can then process the GPS data to determine the location of the device, or if enough processing power is built into the remote tracking device the GPS calculations can be done on the remote tracking device and the derived location information can be transmitted to the central repository. A time stamp can be associated with the location information to provide temporal context for the location information.
An example of such a device is described in U.S. Pat. No. 6,014,080 to Layson, Jr. The remote tracking device of Layson, Jr. includes a tamper resistant strap and housing which holds a GPS engine and a wireless data modem. The remote tracking device communicates with a central station computer using the wireless data modem and transmits the location data for the remote tracking device. The central station includes a computer which is operable to take the position information from the remote tracking device and to compare that location information against a schedule of rules and location restraints to determine if the remote tracking device has strayed from a permitted inclusion zone or has entered a forbidden exclusion zone.
Another remote tracking device is described in U.S. Pat. No. 6,072,396 to Gaukel. The remote tracking device of Gaukel is a two-piece device with a tamper resistant unit securely attached to the person to be monitored. The secure unit is connected to, or in communication with, a body-worn device that includes a GPS engine and a cellular modem. As with Layson, Jr., the cellular modem is used to communicate the location information from the GPS engine to a control station.
Yet another remote tracking device and system is described by U.S. Pat. No. 5,867,103 to Taylor, Jr. The remote tracking device of Taylor, Jr. includes a tamper detection mechanism, a mechanism for receiving a signal from a positioning device, such as a GPS satellite, and a transmitter for transmitting a signal to a central station. The system for monitoring the remote devices includes a position determining mechanism for computationally determining the location of the remote device based on the signal from the positioning device and a temporal marking mechanism for providing a time stamp associated with the location determination.
Each of these devices shares a similar use of GPS and cellular or wireless data technology to gather information about the position of the remote device and to transmit information about the position to a central computer. Further, they all share a similar mechanism for making location determinations and sending those location determinations to the central computer. Prior art devices are scheduled to make location determinations at predetermined intervals and to store that location information and batch send a group of location determinations at a predetermined interval to the central computer.
This mechanism for location sampling and batch sending both provides for a constant drain on battery resources and may result in location information that may be too infrequent in certain circumstances and to frequent in others. For example, location determination at predetermined intervals may be too infrequent for circumstances where the remote tracking device is moving quickly (such as in a car on a highway), or is approaching a boundary of a geographical zone which may be prohibited. That same predetermined interval may be too frequent when the device is stationary for an extended period of time, such as when the wearer is sleeping. By slowing the location sampling frequency in these cases the GPS and cellular engines may be powered down between sampling and sending, thereby preserving battery life.
What is needed is a system and method for changing the frequency between location determinations and/or data transmissions in a remote tracking device.